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Topic: RTL-SDR TCXO Upgrade (Read 2523 times)

Hi, I watched the video of the guy at analogzoo.com about making a 28.8MHz tcxo for the sdr dongle and I'm thinking about building something similar. http://www.analogzoo.com/2016/03/building-a-better-rtl-sdr-tcxo/He basically uses a 19.2MHz 10ppm Crystal where he divides its frequency by two then filters out the third harmonic going out of the divide-by-two chip which is 28.8MHz. He also uses a normal variable capacitor and a negative temp coefficient cap both in parallel with the xtal.My question is that can I just desolder the original, un-temp compensated 28.8MHz crystal from the sdr and just add the two variable caps in parallel?

imho, your best bet will be to buy a dongle which is already powered with a TCXO or CXO clock.Modifying a RTL dongle is really tricky, smd are really small, and I think this is not the best solution.I bought a Nooelec NESDR Smart, not expensive, and I'am very satisfied with its clock performance (miles away from low priced dongles with a simple 28.8M crystal)Save your time and money, buy another dongle.

There are 28.800MHz DIP oscillator modules and 14.400Mhz DIP TXCO modules (which can be passed through a frequency doubler) available on ebay. The signal can then passed through a filter made from the original crystal.I aim to try both methods over the next few weeks.

And yes - I already have 2 x V3 dongles and a Thumbnet N3, but want to tinker with the standard dongles I also have.

If you want a relatively cheap already converted dongle have a look at:

...I bought a Nooelec NESDR Smart, not expensive, and I'am very satisfied with its clock performance (miles away from low priced dongles with a simple 28.8M crystal)Save your time and money, buy another dongle.

There are 28.800MHz DIP oscillator modules and 14.400Mhz DIP TXCO modules (which can be passed through a frequency doubler) available on ebay. The signal can then passed through a filter made from the original crystal.I aim to try both methods over the next few weeks.

Cheers

Thanks but the price for one 28.8MHz tcxo is more than a low cost dongle right?Maybe I'll just buy another one with tcxo and so I can listen to atc and receive adsb at the same time

I just bought "that $20 silver dongle" from amazon aka RTL-SDR V3 with the TXCO (they guaranteed 1ppm stability) and other stuff as well. Its going to arrive next week. I guess it's better than buying the 28.8 Meg TXCO for the same price. @medical-nerd Have you tried the methods you said? I'm interested to know your findings. Thanks

But I would never spend more than a few bucks on one. $20 for the rtlsdr blog version 3 that has all the improvements.. maybe - IF you don't already have one.

Yeah, V3 seems the best as they really thought about the customers and its applications.I guess it will stay for long before V4 as most improvements are done in this version...And PCB space is getting smaller.Have a look at this:

My RTL-SDR arrived. I finally got it! I dont know whats the difference between amazon's standard global shipping and priority shipping which costs a bit more. Anyway I just chose the standard one and it still got delivered in 3 business days to NZ. Thats 7 days less to what they assured me (9-12 days).

Yeah, that's why I bought it. I'm purely interested in its direct sampling ...And then..maybe listen to HF What's interesting is that when I went back to amazon's website for the dongle I just bought, it just said "Currently Unavailable". So I guess I just got lucky right there to grab the last piece in time.

Thanks for pointing that out . That was one of my questions about the dongle..if the dongle had esd protection when it was directly sampled.In the vhf/uhf range though...wel, they said they put an esd rated diode in there.

You should look for the schematic of how its done on the BA5SBA "500 KHz to 1700 MHz" RTLSDR box, because they do a good job there of protecting the input and extracting the signals you want.

Basically you want an LC network band pass and impedance matching network that also puts the input at DC ground potential. There is a differential balanced input o pins 4 and 5 and you want them to be the secondary of an 1:4 RF transformer so twice as many turns - also center tapped, with the center grounded through a capacitor. The input needs to be low pass filtered which can be minimally done by an L and two Cs but you can make a better one with a little thought put into it, knowing what signals you want to go where. I am just waking up and my notes are on my other computer. The discussion about the BA5SBA board is all in Chinese on the HelloCQ web site. (here is his user page http://www.hellocq.net/forum/u.php?uid=120 or https://shop107578303.world.taobao.com/?spm=a1z10.3.0.0.cuLaul ) You need Google Translate.

You should make a decent filter, its easy and definitely worth it. Give me a bit of time, I'll add more info here. You can probably find the schematic for his board - which has an image of a dolphin jumping on it - online - I found it on the hellocq site. But that design could also be improved a lot.

But I would just copy it as a start. It works pretty well. But I would experiment on the RF transformer, it seems the transformer could be much better if you were willing to invest in calculating an optimized RF transformer.

The two pins 4+5 of the RTL chip are a balanced differential input and quite sensitive to ESD damage.

Be careful about ESD!

You can use a transformer to isolate the chip inputs from the antenna.

See the pictures below and the schematics posted higher up in this thread.

This approach should work with all of them, see how it was done above with a generic straight from the factory, Newsky TV28Tv2 dongle which as far as this mod is concerned is electrically identical to the ones sold by rtlsdrblog, cosycave, etc.

(although I think the other midsized one works better in this context because it has better grounding for layout-related reasons, all four corners of the board being grounded, which is important to reducing HF noise. Also it uses the better crystal out of the box, and you don't have to pay extra for it.)

How you connect it can make a very big difference in the number of spurs you receive on lower frequencies.

You really need good bypassing, lots more bypass caps. The voltage regulation has to get major upgrades over what they come with for the dongles to work acceptably doing direct sampling at lower frequencies.

So I gather that the two unpopulated pads are the Q+ and Q-? And I should use them both with a transformer. Can I measure the input impedance directly with a VNA?

I did not see your images before, so now I see what is under the glue...

It seems to me my older "dolphin" board with the filtering and transformer work about the same as my crude direct injection. Neither one has really picked up much of anything below HF other than than the broadcast band; however, I did catch a few CW beeps and sideband mumbling on 80m with the NESDR.

The thing you need is to systematically notch out or filter out all out of band interference. The way the RTLSDRs are designed (for video) makes that necessary.

You need to analyze the interference and tackle it bit by bit. Another approach -maybe a better approach, actually, is using something like a magnetic loop antenna which simply only "works" on the target frequency.

For lower frequencies use longer wires or more loops around the circle, and more capacitance. That is actually by far the simplest approach.

Most antennas work badly if at all on really low frequencies.

A loop is much better than most of them. Another antenna that works well on low frequencies is the PA0RDT Mini whip type ("e-field probe") antenna.

It's my understanding that one can use most computer's sound cards as an input to receive VLF directly, but I have never done that.

I have had 2 of the original version Wellbrook ALA1530 loops for over 10 years, one inside and one outside. They completely transformed my shortwave reception compared to an untuned longwire antenna. I have also used antenna tuners with these successfully but it is not really required.The first version was £125 for the outside aluminium version and are available sometimes on ebay but quickly get snapped up. I would recommend reading the reviews to give an indication as to how useful loops can be.There are very good examples of tuned homebrew loops in the on-line literature that can be built for minimum cost with a vintage variable capacitor.

Thanks for reminding me, I have a loop antenna that I built into a x-tal radio. I also found this site useful and further down is a link to a site detailing the Wellbrook, including a complete teardown, x-rays of the amplifier module, and schematic. http://www.g8jnj.net/activeantennas.htm

Its so extremely easy to make a loop. Definitely the best HF antenna for somebody with limited space.

Vacuum variable capacitors would definitely be the way to go. Surplus military units can be had for between $60 and $100 and seem worth it when you consider the importance of having a low resistance for the loop.

The bushings on more conventional variable caps can eat up a lot of the gain by reducing the Q.

Most variable caps will do fine.. Depending on what size cap you have, what frequency you want to receive, and how many loops your string, most loops of wire from 18 to 20 inches or so up to a few meters in diameter - when put in series with your capacitor should be able to resonate somewhere in HF - Connect a small pickup loop to a length of coax, put it in the plane of your big loop, and experiment by tuning the capacitor up and down. Keep in mind that the peak may be very narrow so it might be easy to miss it. Set your SDR to view as wide of a range as possible, (my computer does fine with 2.8 Mhz sample rate) and then set the SDR to some frequency near where you expect it to resonate..

tune in an area where there are normally a lot of signals, and tune up and down with the cap. Look for something that looks like a wave of strong signals.

The area of strong signals may be as little as 30 or 40 kHz wide or it may be much wider. The better a connection you have electrically- the lower the resistance in the system, the sharper it may be.

Its sometimes worth it to clean your variable caps to get the best connection.

Antenna is 12 next to 3 turns, and dual gang AVC, with 400 pf on the main coil and 200 pf coupling the coil. It tunes the MW band nicely and you can see the hump sweep just beyond the ends of the band when tuning.

It does nothing everywhere else. I tried tapping it at various spots, opening it up, putting the coils in series, and adding another 1000pf AVC in various ways. All I could accomplish was MW station images. I'll try to build something else and filter out the MW band.